Among various technologies known in the art for the manufacture of seamless steel pipes, the most efficient methods suited for mass production are those manufacturing methods which are based on a rolling technique (the so-called Mannesmann process) comprising piercing billets using a pair of inclined rolls and a piercing plug.
In piercing by the rolling technique, a heated billet is transferred to a piercing machine (piercer), pushed by a pusher and gripped by a pair of inclined rolls. Thereafter, the billet advances while the rolls rotate it. On that occasion, a rotary forging effect (Mannesmann effect) is exerted on the central portion of the billet during the period until the billet arrives at the tip of the piercing plug disposed along the pass line between the rolls and, as a result, that central portion becomes fragile. Then, the billet undergoes piercing-rolling by the pair of inclined rolls and the plug to form a hollow raw pipe (hereinafter also referred to as “raw pipe” for short). The hollow raw pipe is further processed in the subsequent steps, including elongation rolling, to give a seamless pipe having a predetermined size.
The piercing-rolling mentioned above is also applied to billets made of, for example, a continuously casting material having center segregations and/or porosity or of a stainless steel species poor in hot deformability. On that occasion, the rotary forging effect and additional shear deformation cause formation of leaf-like, fin-like or lap-like flaws or defects on the inner surface of the hollow raw pipe (such defects are collectively referred to as “inner surface defects”). For preventing this, the plug tip draft is generally reduced to thereby suppress the rotary forging effect as far as possible and prevent the formation of such inner surface defects. However, the reduction in plug tip draft tends to allow the occurrence of misrolling such as a gripping failure.
The plug tip draft is defined by the following formula:(Bd−d1)/Bd, namely 1−(d1/Bd)
Therefore, to reduce the plug tip draft means that when the billet diameter (Bd) is constant, d1 (gap between the rolls at the plug tip position) is increased or that the plug is shifted forward to the billet side to shift the tip thereof in the direction toward the smaller roll diameter side (cf. FIG. 1).
Patent Documents 1 and 2 describe methods of manufacturing seamless pipes one feature of which is to employ a plug tip draft of not lower than 95% or not lower than 97%. However, while these documents define the plug tip draft as “gap between the rolls at the plug tip position/billet diameter”, the above-cited “not lower than 95%” and “not lower than 97%” should properly be described as “not lower than 0.95” and “not lower than 0.97”, respectively. And, these plug tip draft ranges correspond to “not higher than 0.05” and “not higher than 0.03”, respectively, according to the original definition given above.
[Patent Document 1] Japan Patent Unexamined Publication No. 2001-162307 (Application No. H11-346513)
[Patent Document 2] Japan Patent Unexamined Publication No. 2001-162306 (Application No. H11-346514)
Another difficulty arising from such reduction in plug tip draft consists in a decrease in piercing efficiency. The piercing efficiency is the percentage of the longitudinal advancement velocity of the raw pipe to the longitudinal direction component of the roll gorge peripheral velocity and is defined as follows:η=(VH/VR sin θ)×100(%)where η is the piercing efficiency (%), VH is the longitudinal velocity of the raw pipe (m/s) and VR is the roll gorge peripheral velocity (m/s).
FIG. 4 shows the results of tests carried out for piercing efficiency investigations using plugs of the same shape under the conditions shown in Table 1. As shown, the piercing efficiency decreases with the increase in plug tip draft and, in particular, the decrease in piercing efficiency is remarkable when the plug tip draft is 0.04 or lower.
[Table 1]
TABLE 1Billet material and sizeS45C, SUS304, diameter: 70 mmHollow raw pipe sizediameter: 75 mm, wall thickness: 8 to 9 mmRoll gorge diameter410 mmToe angle γ15°Feed angle β10°Plug tip draft TDFT0 to 0.05Maximum plug diameter 58 mmDistance from plug tip to115 mmmaximum diameter site
A reduction in piercing efficiency means a decrease in the longitudinal velocity of the raw pipe (above-mentioned VH) or, in other words, a decrease in the longitudinal velocity of the billet and means that the time during which the rotary forging effect is exercised on the billet is prolonged (the number of times of roll forging at the predetermined position of the billet is increased). Thus, with steel species having defects at the central portion, for example continuously casting materials, an excessive rotary forging effect even if the plug tip draft is set at a low level causes inner surface defects.
Furthermore, as a result of a decrease in piercing efficiency, the metal flow of the rolling target material is restrained in the axial direction and is facilitated in the circumferential direction. Then, the additional shear deformation in the circumferential direction increases, and the defects produced in front of the plug are further intensified by that shear deformation and, as a result, they remain on the raw pipe as large-size inner surface defects. In addition, the time required for piercing is prolonged by the decrease in piercing efficiency, so that a further problem arises, namely the thermal load on the plug increases and the life of the plug is shortened.
The methods described in the above-cited Patent Documents 1 and 2 both consist in combining a reduced roll circumferential velocity with pushing in by means of a pusher. According to these methods, piercing is carried out at a low plug tip draft even in the piercing of the middle portion of the billet, so that cracking due to the rotary forging effect in front of the plug can indeed be suppressed. However, under certain conditions of setting of the rolls and according to the shape of the plug, the slip in piercing the middle and subsequent portions of the billet increases and, as a result, the piercing efficiency may be decreased thereby, although the problem of gripping failure can be solved.
If the piercing efficiency in piercing the middle and subsequent portions of the billet is decreased, as mentioned above, the velocity, in the direction of rolling, of the entry side billet is decreased even in the steady rolling region, the number of billet revolutions (number of times of contacting between the roll pair and the rolling target material during the period from gripping of the billet by the rolls to the arrival of the billet at the plug tip) increases. Therefore, the number of times of the billet experiencing the rotary forging effect increases and, even if the plug tip draft is lower, cracks will be caused in the vicinity of the billet center by the excessive rotary forging effect and thus remain as inner surface defects in the raw pipe.